1. Field of the Invention
The present invention relates to a radiation imaging system which takes in incident radiation distribution and a nuclear medicine diagnosis instrument therefore. More particularly, the present invention relates to a technique for achieving accurate diagnosis.
2. Description of the Related Art
A diagnostic method using a single photon emission computed tomography system (abbreviated to “SPECT instrument”), among nuclear medicine diagnosis instruments using radiations for medical diagnosis, gives a radioactive medicine to a subject, namely, a patient, measures the distribution of the radioactive medicine in the subject's body by using images projected on a plurality of planes at different angles and produces a tomographic picture from those images. This method can detect functions on the level of molecules and metabolism and can provide a tomographic picture based on physical functions.
The SPECT brings a plurality of radiation imaging devices from predetermined directions near to a predetermined part of the subject, such as the heart or the head, revolves the radiation imaging devices about the body axis of the subject to obtain minute information about a specific organ and a specific function. High-order diagnosis can be made on the basis of the thus obtained minute information.
It is desired to produce a picture of a high picture quality (high resolution) to achieve advanced diagnosis accurately. Since the closer the radiation imaging devices to the subject, the higher is the resolution, it is desired to diminish a space surrounding the effective field of view of each radiation imaging device to dispose a plurality of radiation detecting surfaces close to the subject.
The following measures are taken to improve image quality by improving spatial resolution. A pixel detector is employed for such a purpose. The pixel detector has a detecting unit formed by integrating a plurality of radiation detecting elements capable of detecting pixels of a picture projected on a plane in a high density. This pixel detector improves intrinsic spatial resolution. Resolution can be remarkably improved particularly in closer imaging. Use of the pixel detector diminishes a space needed by a photomultiplier tube extending outside the detecting surface (effective field of view) because a signal read system can be disposed on the back surface of the detecting surface (effective field of view).
A collimator is mounted in front of the detecting surface. The collimator regulates incident directions from which radioactive rays radiated from the subject and falling from different directions on the detecting unit. The collimator of the pixel detector needs to be provided with a plurality of through holes determining the incident directions of radioactive rays at positions coinciding with the radiation detecting elements. As mentioned in JP-A 2006-119113, a moiré pattern is produced in a projected picture if the through holes are dislocated from their correct positions.
A method of correcting the incorrect positional relation between the through holes and the radiation detecting elements mentioned in JP-A 2001-324569 adjusts the position of the collimator relative to a box holding the detecting unit by an adjusting screw.
The incorrect positional relation between the through holes and the radiation detecting elements causes not only formation of a moiré pattern, but also the deterioration of resolution and the reduction of linearity of a picture. Accuracy of the positional relation between the through holes and the radiation detecting elements needed to ensure the linearity of the picture is still higher than that needed to solve the problem of a moiré pattern.
In a detecting unit for obtaining a tomographic picture of high resolution, radiation detecting elements are arranged at pitches in the range of about 1 to 2 mm. Under such a condition, an allowable error in the positions of the through holes relative to the corresponding radiation detecting elements needed to avoid the foregoing problems is in a narrow range of 0.1 to 0.2 mm.
If the pixel density is increased by reducing the size of pixels to elevate the accuracy of diagnosis, the collimator needs to be installed in a high mechanical accuracy.
The box holding the detecting unit of a radiation imaging system using radiations, such as γ-rays, for obtaining an image is required to have a radiation screening function also. Therefore the radiation imaging system often has a large size and a large weight.
When machining accuracy is taken into consideration, it is difficult to confine positional errors of the component parts of the radiation imaging system to the foregoing range of allowable errors. Such errors in the position of the component parts may be reduced by a position adjusting means for adjusting the positional relation between the detecting unit and the collimator after installation.
When the radiation imaging system is provided with a position adjusting means for adjusting the position of the heavy collimator, the radiation imaging system needs a large, heavy, movable mechanism, the detector box is practically enlarged and the merits of closer imaging are nullified.